This invention relates generally to a semiconductor laser device which is capable of generating a visible ray at an increased efficiency and a method of making the same. More specifically, this invention relates to a semiconductor laser device which is essentially made of a substance of an indirect-transition type band structure, inherently having a very poor capability of radiation, such substance being modified by ionimplantation of nitrogen atoms, and which device is so constructed as to cause the flow of bias current to converge to the nitrogen-implantation part of the device, thus generating a visible laser at an elevated efficiency.
Almost all the laser devices hitherto known radiate infrared rays, which are of relatively long wavelength. This is attributable to the use of GaAs and other semiconductor materials of the chemical compounds of the III-V families which, although known as optoelectronics materials of high luminescent performance convenient for use in light-emitting diodes, semi-conductor lasers and other luminescent devices, have a band structure of narrow energy gap.
In order to cause the radiation of a visible laser from a chemical compound of the III-V families it is necessary to select a material of large energy gap (2 or more electron volts). Almost all the materials that meet this requirement are of indirect-transition type band structure and therefore cannot generate luminescence at an elevated efficiency. There are two different solutions to improve the efficiency of radiation.
One is to select a material which readily emits luminescence in spite of its small energy gap and add to the so-selected material a proper amount of indirect-transition type semiconductor material of a large energy gap, thus converting the material to a direct-transition type semiconductor. This remedy, however, disadvantageously limits the selection of a substrate to be modified and the ratio of mixed crystal.
The other solution is to put a luminescence center-constituting impurity in a crystal which may be of relatively low luminescent performance, thus converting the material to a substance of high luminescent performance. Indeed, light-emitting diodes made of GaP doped with nitrogen atoms have been produced and used. Also, laser oscillation has been caused by the agency of nitrogen atoms in GaAs.sub.1-x P.sub.x. The inventor doped an Al.sub.x Ga.sub.1-x As with nitrogen atoms according to the ion-implantation method, with a view to allowing the atoms to behave as isoelectric impurities in the crystal and to constitute an effective luminescent center. The inventor observed the visible laser from the so-made semiconductor laser device and determined the luminescent efficiency to be nearly 1000 times as large as the conventional indirect-transition type luminescent semiconductor.
No impurities other than nitrogen atoms have been hitherto known and used for optically activating an indirect-transition type semiconductor material so as to increase the luminescent efficiency. Also, no method other than the ion-implantation method has been hitherto known and used for doping a semiconductor material with nitrogen atoms. In the course of ion-implantation of impurities, however, the semiconductor material will be inevitably exposed to the atmosphere, thus causing an oxide film to appear on the material, and the soformed oxide film will prevent another layer from growing on the semiconductor, thus making it difficult to build a heterojunction structure. Still disadvantageously, in the activation layer which has been formed by ion-implantation there remains some defects in the crystal lattice caused by the ion-implantation which cannot be completely reduced by annealing. The remaining defects will become centers of non-radiative recombination and adversely reduce the optical activation. As a consequence the requirements for laser oscillation will become difficult to satisfy, and the threshold value of laser oscillation will disadvantageously rise. A solution to this problem is to invent a new laser structure having a layer capable of confining carriers and hence photons therein in such a way that the concentration or density of photon-carriers rises in an activation zone.
One object of this invention is to provide a semiconductor laser device which has a carrier-confining layer, and is capable of generating a single mode of visible ray, excited above a relatively low threshold value of laser oscillation, at a luminescent efficiency 1000 times as large as a conventional indirect-transition type semiconductor.
Another object of this invention is to provide a method of making the same.